The surfaces of window glasses for buildings and vehicles are provided with a transparent conductive film for reducing the total solar light transmittance while maintaining high visible light transmittance for light that enters from outdoors, and for reflecting infrared rays that enter from the interior of rooms or vehicles. The transparent conductive film contains a metal oxide such as tin oxide, titanium oxide, zinc oxide, or indium oxide as its main component, and is usually doped with fluorine or the like in order to enhance its conductivity. Photoelectric conversion devices also use glasses provided with a similar transparent conductive film. For example, a glass provided with a transparent conductive film is used in a photoelectric conversion device provided with a thin film of non-crystalline silicon or microcrystal silicon as its photoelectric conversion layer, to serve as a substrate glass on which the above thin film is to be adhered. In these photoelectric conversion devices, a transparent conductive film is formed between the glass and the photoelectric conversion layer, and further on the photoelectric conversion layer as needed, so that it functions as a thin film electrode for taking out electrons and holes generated in the photoelectric conversion layer. In addition, in order to increase the photoelectric conversion efficiency, it is necessary to guide a larger amount of light into the photoelectric conversion layer, and it is required that the transmittance for visible light and near-infrared rays is high (the reflectance is low).
This transparent conductive film contains, as described above, a metal oxide as its main component, and its refractive index is about 1.8 to 2.6, which is higher than the glass. For reference, the refractive index of ordinary glass made of a soda-lime composition is about 1.5. When a transparent conductive film is formed directly on a glass surface, the adhesiveness of the transparent conductive film tends to be insufficient because of the difference in the thermal expansion coefficients and crystal morphologies, and a problem arises that the reflectance becomes high due to the high refractive index of the transparent conductive film. Moreover, in the case of glass containing a large amount of alkaline component, such as a soda-lime glass composition, another problem arises that the alkaline component diffuses into the transparent conductive film over time, reducing the conductivity of the transparent conductive film, or reducing the adhesiveness thereof
In order to solve such problems, the present inventors developed a technique for improving the adhesiveness of a transparent conductive film by forming an undercoating film made of a first layer containing a metal oxide as its main component and a second layer containing silica (SiO2) as its main component, stacked in that order from the glass side, between a glass and a transparent conductive film, and by providing roughness of an appropriate size on the surface of the first layer, on which the present inventors already filed a patent application (JP 2000-261013A).
In addition, in view of the fact that the surface roughness of a transparent conductive film can be enlarged by enlarging the surface roughness of an undercoating film, the present inventors also developed a technique for forming through holes partially in the first layer of the undercoating film, in order to enhance the light trapping effect (the effect that causes the optical path length in the photoelectric conversion layer to lengthen by scattering the incident light) by enlarging the surface roughness of the transparent conductive film, on which the present inventors already filed a patent application (JP 2001-53307A). Furthermore, focusing on the fact that a refractive index-varying layer, in which the refractive index gradually changes, is formed in the surface roughness of this transparent conductive film, the present inventors also developed a technique for reducing the reflectance in the surface of the transparent conductive film by controlling the state of change in the refractive index, on which the present inventors already filed a patent application (JP 2001-48593A).
However, according to the invention described in JP 2000-261013A, the roughness is formed only on the surface of the first layer of the undercoating layer, which has a thickness of several nanometers; therefore, the size of the roughness is not necessarily sufficient and there is room for improvement in terms of increasing the adhesiveness of the transparent conductive film.
Furthermore, according to the invention described in JP 2001-53307A, although through holes are formed in the first layer of the undercoating film, these through holes dispersedly exist in places. Therefore, although the surface roughness of the transparent conductive film is large in the areas directly above the through holes, such a large surface roughness cannot be formed over the entire surface of the transparent conductive film and there is also room for improvement in enhancing the light trapping effect.
Further, the invention described in JP 2001-48593A focuses only on the refractive index-varying layer of the transparent conductive film and no research was conducted about the reflectance at the interface between the first layer and the second layer of the undercoating film, and at the interface between the undercoating film and the transparent conductive film. Thus, in this invention too, there is room for improvement in the reduction of the reflectance at the above-noted interfaces.